In general, in digital recording, information is encoded in the spacing of magnetic polarity transitions. During the reading process, the polarity transitions are converted into voltage pulses, so that the information in the read signal is encoded in the spacing of pulse peaks. Therefore, for data integrity, the accuracy of the timing of the peaks in the read signal is critical. There are, however, inherent distortions of the read signal that must be accommodated.
Binary information on magnetic flexible disks is stored by magnetizing small areas of the magnetic surface with one of two polarities. For most flexible disk drives used in personal computers, the small magnetized areas are recorded primarily longitudinally (in the plane of the surface). Although the magnetized areas are primarily longitudinal, they also have an unavoidable partial vertical component (perpendicular to the surface). During reading, a magnetic coil in a read head intersects magnetic fields from the small magnetized areas. As the head passes through the fields, a transition from one polarity to the opposite polarity results in a changing field that in turn induces a changing current in the coil of the read head. The changing current typically drives a resistive load to provide a voltage signal. Ideally, for purely longitudinal magnetization, an isolated transition from one polarity to the opposite polarity results in a perfectly symmetrical voltage pulse. Ideally, for purely vertical magnetization, an isolated transition from one polarity to the opposite polarity results in a perfectly symmetrical dipulse (a pulse of one polarity followed by a pulse of the opposite polarity). The combined longitudinal and vertical components result in an asymmetrical voltage pulse. The asymmetry is sometimes referred to as a peak shift. As bit densities increase, the transitions are no longer isolated. In particular, if two adjacent write transitions are very close together, the combined effects of peak shift and adjacent pulses make the time between read pulses on read-back longer than the time between the transitions during writing.
Given a particular digital pattern in the write waveform, it is possible to predict some of the resulting distortion in the read waveform. It is then possible to distort the writing signal to "precompensate" for distortion in the read process. For example, if two adjacent write transitions are very close together, then the first transition may be written "late" and the second transition written "early." As a result, the time between transitions during writing is shorter than the "ideal" time and the time between pulses during read back is equal to the "ideal" time. In many flexible disk controllers, this is done only on inner tracks where bit densities are highest.
Within a personal computer, the flexible disk controller may be used to control not only multiple flexible disk drives but also may be used to control a data recording tape drive. Precompensation by advancing and delaying polarity transitions is appropriate for flexible disk drives but may not be appropriate for tape drives sharing the same controller. As hard disk capacity increases, there is a need for tape back-up drives having substantially higher bit densities than flexible disks. For these higher bit densities, precompensation designed for flexible disks has a substantial negative effect on data integrity. In many computer systems, however, precompensation is permanently enabled in hardware and cannot be turned off by software drivers. If a high density tape drive is used with a flexible disk controller with automatic precompensation that cannot be turned off, the tape drive needs to be able to detect the precompensation and remove it.